Software RAID within RAID

Software RAID within RAID

[David Anderson]

Hi all,

First of all I would like to thank the developers of the linux software
RAID system for their excellent work which has saved my data on several
occasions.

Recently I lost one of the disks on my software RAID5 (3x2Gb SCSI on a
2.4.21 Debian Woody). Seeking replacements, I came across two 4Gb SCSI
drives. After inserting one of them as a temporary replacement for the
dead 2Gb, I am now looking into ways of better integrating these two new
disks into my existing setup.

The most best solution I came up with was to extend the array's overall
capacity from 4 to 8Gb. To do this I would first combine the two
remaining 2Gb disks into a RAID0 array, and then combine that array and
the two 4Gb disks into a RAID5 array. To clarify this, here is a diagram
of this solution: http://polaris-outpost.net/vrac/Solution.png .

However, there are a few things I would like to know before I make an
attempt at this setup. First of all, would the software-raid driver
handle disk failures properly even with the RAID within RAID? I do not
see why not, but I'd rather be sure before I jump...
The other question I have is about performances. Would the RAID0 array
significantly impact on the overall I/O perfomances of the RAID5 array?
If anyone has already had the opportunity of comparing performances on a
regular RAID5 and a RAID5 containing a RAID0, I would be interested in
the results. I will of course conduct my own tests once this setup is in
place.

Thanks in advance for your help
David Anderson

Re: Software RAID within RAID

[Neil Brown]

On Friday July 25, david.anderson@calixo.net wrote:
> Hi all,
> 
> First of all I would like to thank the developers of the linux software
> RAID system for their excellent work which has saved my data on several
> occasions.
> 
> Recently I lost one of the disks on my software RAID5 (3x2Gb SCSI on a
> 2.4.21 Debian Woody). Seeking replacements, I came across two 4Gb SCSI
> drives. After inserting one of them as a temporary replacement for the
> dead 2Gb, I am now looking into ways of better integrating these two new
> disks into my existing setup.
> 
> The most best solution I came up with was to extend the array's overall
> capacity from 4 to 8Gb. To do this I would first combine the two
> remaining 2Gb disks into a RAID0 array, and then combine that array and
> the two 4Gb disks into a RAID5 array. To clarify this, here is a diagram
> of this solution: http://polaris-outpost.net/vrac/Solution.png .
> 
> However, there are a few things I would like to know before I make an
> attempt at this setup. First of all, would the software-raid driver
> handle disk failures properly even with the RAID within RAID? I do not
> see why not, but I'd rather be sure before I jump...

Yes.  This definately works.  
Though it wouldn't hurt for you to simulate a failure just to convince yourself.

> The other question I have is about performances. Would the RAID0 array
> significantly impact on the overall I/O perfomances of the RAID5 array?
> If anyone has already had the opportunity of comparing performances on a
> regular RAID5 and a RAID5 containing a RAID0, I would be interested in
> the results. I will of course conduct my own tests once this setup is in
> place.

I haven't tested it myself (though I have a setup rather like this
running at the moment: used to 36Gig drives while waiting for a
failed 72Gig drive to be replaced).

NeilBrown

> 
> Thanks in advance for your help
> David Anderson
> -
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